The phrase "Salad Days" refers to a time when a person is at the peak of his abilities.
Example of use: Mark -"When I was young I went out every night, came back home at 4 in the morning, slept for 3 hours and went to work feeling awesome". Ron-"well those were your salad days. You're too old for that today"
The phrase was coined by William Shakespeare in the tragedy 'Antony and Cleopatra' in 1606. In those days the meaning of the phrase was foolish and carelessness behavior due to lack of experience. However, the modern use the meaning changed, and now the phrase describes a period when a person is at the peak of his abilities.